1. Technical Field
The present invention relates to a display device using a current driving type light-emitting element typified by an organic EL (Organic Light Emitting Diode: OLED) element, and to a method for manufacturing the display device.
2. Background Art
In recent years, the interest in FPDs (Flat Panel Display) has been increased, instead of conventional CRTs (Cathode Ray Tube). As typical FPDs, LCDs (Liquid Crystal Display) and PDPs (Plasma Display Panel) have been already put into practice. However, LCDs and PDPs are known to have problems as mentioned below.
More specifically, LCDs themselves emit no light, thus require a high-luminance backlight, and tend to increase the electrical power consumption. Furthermore, LCDs fall short of CRTs in terms of viewing angle and response speed. On the other hand, PDPs themselves emit light, and are comparable to or better than CRTs in terms of viewing angle and response speed. However, PDPs require higher voltages for driving, and have the problem of difficulty in reduction of electrical power consumption.
Besides, in recent years, attention has been paid to organic EL as a candidate for a next-generation FPD. The organic EL has the possibility of solving the problems described above, which are problematic in LCDs or PDPs.
An organic EL device is typically manufactured in accordance with a method as mentioned below.
(a) First, an anode is formed on a support substrate such as cleaned glass, quartz or plastics, and subjected to patterning. For the anode, an Indium Tin Oxide (ITO) which has a large work function is typically selected. The anode is usually formed by sputtering.
(b) Then, a hole injection layer and an organic EL layer are formed. In general, the layers are formed by a vacuum deposition process in the case of low molecular weight organic EL, or formed by a spin coating process of an ink-jet process in the case of high molecular weight organic EL. The ink-jet process is selected when it is necessary to apply organic EL in different colors. It is to be noted that a hole injection layer, a hole transporting layer, an electron transporting layer, an electron injection layer, etc. may be formed before or after forming the organic EL layer in order to improve the luminous efficiency and the lifetime. In the case of high molecular weight organic EL, a hole injection layer and a hole transporting layer are usually formed by an ink-jet process.
(c) Then, a cathode including a metal such as Al is formed by a vacuum deposition process or the like, and sealing is carried out.
The steps described above complete a device. Such a device is referred to as a bottom-emission device because the device has a structure in which light is extracted from the anode side.
There are two types of organic EL display includes, an active matrix type and a passive matrix type. Active matrix type organic EL displays suitable for large screens require at least two thin film transistors (Thin Film Transistor: TFT) for each pixel. Furthermore, more several TFTs are required in order to compensate TFT characteristics and manufacture uniform displays, and the bottom-emission device as described above thus have the problem that the opening is limited by the arrangement of TFTs, thereby decreasing the aperture ratio.
In order to solve this problem, top-emission devices in which light is extracted from the cathode side have been actively researched and developed in recent years. In the top-emission devices, higher aperture ratios can be achieved without being affected by the number or size of TFTs. This is because long-life displays can be achieved as a result. The top-emission devices may have an anode layer of a reflective metal such as silver or a silver alloy subjected to ozone treatment, or may have an anode layer of the reflective metal on which, for example, an ITO with a large work function is formed.
In the top-emission devices, the cathode needs to be transparent since light is extracted upwardly. While common transparent electrodes include an ITO, the ITO is disadvantageous in that its resistance is high. Therefore, as the display size is increased, the voltage drop in the cathode is increased, thereby resulting in the problem that unevenness in luminance is caused in light emission to decrease the display quality of the display.